The most widely used infrared absorbers are black metal layers (e.g. Au, Ag, Pt), produced by deposition under a given foreign gas pressure. The very high and uniform infrared absorption is obtained through the porosity of the layers. The disadvantage of these absorber layers is their sensitivity to mechanical and chemical influences, so that the production of a clearly defined absorber geometry using CMOS technology processes is only possible by deposition via a resist mask, which leads to limitations in the technology. Another process for producing metal layers is electrodeposition. The problems of this process are inter alia ionic impurities of the electrolyte, the stressing of the disk-like support of the metal layers (e.g. silicon wafer) during electrolysis and that auxiliary planes are required for the electrodeposition of films. At present this is not a CMOS technology standard.
The literature refers to the possibility of infrared absorption by thin metal layers and .lambda./4 layers (e.g. A. Hadni and X. Gerbaux, Infrared and Millimeter Wave Absorber Structure for Thermal Detectors, Infrared Phys., vol. 30, No. 6, 1990, pp 465-478). By means of thin metal films it is possible to produce a uniform absorption for wavelengths above 1 .mu.m, but which amounts to max 50%. The reflection losses on the absorber surfaces can be reduced with antireflection layers (.lambda./4 layers). However, the absorptivity of the thin metal layers is very dependent on the layer thickness, so that a precise layer thickness control is necessary for the production thereof.
Another group of infrared absorbers consists of organic layers such as hydrocarbon blacks and so-called black varnishes. At present, there are no CMOS-compatible technologies for the deposition and structuring of these layers.
H. Saha et al, Influence of Surface Texturization on the Light Trapping and Spectral Response of Silicon Solar Cells, IEEE Transactions on Electron Devices, vol. 39, No. 5, 1992, pp 1100-1106 describes an absorber structure, whose surface has pyramidal grooves or depressions formed by anisotropic etching in monocrystalline silicon. The disadvantage of this structure is that, firstly, a monocrystalline silicon layer must be applied for the production thereof, which is a process step requiring additional technological expenditure.
U.S. Pat. No. 4,620,364 discloses solar cells, which have a central monocrystalline layer (e.g. silicon crystal), as well as an upper and a lower, non-absorbing layer, both of which can be structured. By multiple reflection of the infrared radiation between the upper and lower layers, the optical path length of the radiation in the absorbing central layer is increased and, consequently, the solar cell efficiency is improved.
Such an arrangement cannot be appropriately applied to a support body (e.g. a membrane), as would be necessary for use as a thermal detector. As stated hereinbefore, increased technical expenditure is involved in the application of a monocrystalline layer, e.g. of silicon.